The present invention relates to a heat exchanger tube which is used in ethylene cracking furnace or other tubular heat furnaces to increase the efficiency of the heat transfer. Especially, the present invention relates to an ethylene cracking furnace or other tubular heat furnaces. The present invention further relates to a method for making the heat exchanger tube according to the present invention, i.e., making the heat exchanger tube with a twisted baffle integrated with its inner surface by means of smelting the raw material in the vacuum condition and precision casting with the model being burning away.
As those skilled in the art know, the key to increase the output of some chemical products such as ethylene and propylene is to increase the temperature for cracking and shorten the time of the raw materials staying in the furnace tubes. For this purpose, the efficiency of the heat transfer of the furnace tubes must be tried to be increased.
In this way, some manufactures in the world have used a kind of furnace tube, the inner surface of which are integrally provided with a plurality of spiral ribs, the cross section of this kind of furnace tube is shaped as a plum blossom. The central portion of this kind of furnace tube is hollow. In this construction, the area of the inner surface of this kind of furnace tube is increased, so the area used for heat transfer is increased. For this reason, the efficiency of the heat transfer of the furnace tubes is also increased. However, inside this kind of furnace tube, the flowing speed in the central portion is much faster than that on the inner wall, resulting in a marked difference in temperature between the central portion and the inner wall. This in turn will make it more possible to result in not fully cracking and begin coking.
In recent years, those skilled in the art have been trying to find a technical solution which can increase the area used for heat transfer so as to increase the efficiency of the heat transfer of the furnace tubes while minimizing the differences between the flowing speed in the central portion of this kind of furnace tube and that on the inner wall thereof, and minimizing the difference in temperature between the central portion and the inner wall, so as to make more fully cracking inside the furnace tubes. Chinese Utility Model CN 87 2 03192U discloses a baffle construction for increasing the efficiency of the heat transfer, comprising a heat exchanger tube and a twisted baffle. Said twisted baffle is fabricated from an elongated generally rectangular sheet of flat metal. On each of the pair of straight parallel side edges in length of the raw sheet of metal are provided with a plurality of rectangular teeth. The flat sheet of metal with rectangular teeth on its side edges in length is twisted to form said twisted baffle. Such twisted baffle is inserted into said heat exchanger tube. It is clear that only the two ends of the twisted baffle can be welded on said heat exchanger tube. The Chinese Utility Model can make more fully cracking inside the furnace tubes. Unfortunately, the middle portion of the twisted baffle can not be welded on the inner wall of said heat exchanger tube. As the flowing speed of the raw material is fast enough to impact heavily on the twisted baffle inside the heat exchanger tube, making such shaped baffle being subject to a strong vibrating excitation, so as to be damaged easily. Moreover, between the outer profile of the twisted baffle and the inner wall of the heat exchanger tube, and between every two adjacent rectangular teeth on the side edges in length of said twisted baffle, are always generating small eddy. This make it more possible to result in not fully cracking and begin coking.
The object of the present invention is to provide a heat exchanger tube which is used to further increase the efficiency of the heat transfer, make it less possible to begin coking, always work well and be reliability during heat exchanging, and be able to work for a longer time.
Another object of the present invention is to provide a heat exchanger tube, the surface of the twisted baffle of the heat exchanger tube is smooth enough, the inner wall of the heat exchanger tube is finished enough, and the errors in dimension and in geometrical form are small enough.
The further object of the present invention is to provide a method for making the heat exchanger tube according to the present invention, to make the heat exchanger tube with a twisted baffle integrated with its inner surface in a simple, easy and low cost way.
Another object of the present invention is to provide a cracking furnace which uses the heat exchanger tube according to the present invention, not only being able to make the in-process materials go forward while being in a helical motion itself so as to increase the efficiency of the heat transfer, but also not notably making the flowing speed of the in-processing flow slower; not only increasing the output of the desired chemical product, but also lengthening the period to clear the coking.
The further object of the present invention is to provide a tubular heat furnace which uses the heat exchanger tube according to the present invention, being able to increase the efficiency of the heat transfer with low cost and process more in-process materials.
According to the first aspect of the present invention, there is provided a heat exchanger tube, which has at least one twisted baffle therein, each of said twisted baffles extends in the heat exchanger tube along the axis thereof, said twisted baffles extends as long as at least a part of the entire length of said heat exchanger tube, and said twisted baffles are integrated with the inner surface of said heat exchanger tube.
Preferably, the twisted angle of said twisted baffles is between 100xc2x0 to 360xc2x0.
Preferably, the ratio between the axial length of said heat exchanger tube with the twisted angle 180xc2x0 of said twisted baffles and the internal diameter of said heat exchanger tube is 2 to 3.
Preferably, the thickness of said twisted baffles is approximated to that of said heat exchanger tube; in every cross section of said heat exchanger tube, the transition zone from the surface of said twisted baffles to the surface of said heat exchanger tube, and vice versa, is in the shape of a concave circular arc.
Preferably, said the heat exchanger tube with a twisted baffle integrated with its inner surface is made by means of smelting the raw material in the vacuum condition and precision casting with the model being burning away.
According to the second aspect of the present invention, there is provided a cracking furnace tube, which uses at least one said heat exchanger tube according to the present invention, any two of said heat exchanger tubes are separated from each other in at least one section of the radiation heating furnace tube, the distance between the two adjacent said heat exchanger tubes is at least 5 pitches.
Preferably, the distance between the two adjacent said heat exchanger tubes is 15 to 20 pitches.
According to the third aspect of the present invention, there is provided a tubular heat furnace, which uses at least one said heat exchanger tube according to the present invention, any two of said heat exchanger tubes are separated from each other in at least one section of the radiation heating furnace tube, the distance between the two adjacent said heat exchanger tubes is at least 5 pitches.
Preferably, the distance between the two adjacent said heat exchanger tubes is 15 to 20 pitches.
According to the forth aspect of the present invention, there is provided a method for making the heat exchanger tube according to the present invention, which includes the steps of smelting the raw material in the vacuum condition and precision casting with the model being burning away, wherein the model used for forming the twisted baffle is composed of a plurality of parts, a profile in conformity with the surface shape of said twisted baffle is formed when combining every parts of said model together.
According to the technical solutions, when the in-process materials pass through the surface of said twisted baffle inside said heat exchanger tube, said twisted baffle directs the in-process materials away from the center of said heat exchanger tube, flowing forward helically other than straight ahead, so that the in-process materials passing through inside said heat exchanger tube flows laterally while going ahead, so as to strongly spray onto the inner surface of said heat exchanger tube. In this way, the thickness of the boundary statically flowing layer on the inner surface of said heat exchanger tube becomes much thinner, so that the heat resistance of the boundary layer on the inner surface of said heat exchanger tube is much smaller, therefore, the efficiency of the heat transfer of said heat exchanger tube can be increased.
As the efficiency of the heat transfer of said heat exchanger tube is increased, the temperature on the inner surface of said heat exchanger tube is lowered accordingly. This in turn make it more possible to prevent coking, so as to further increase the efficiency of the heat transfer of said heat exchanger tube.
According to the present invention, each helical passage defined by the inner wall of the heat exchanger tube and the surface of the twisted baffle is smooth and finished enough, forming no dead space for resisting the flow of the in-process materials. With this reasons, it is more possible to prevent coking and further increase the efficiency of the heat transfer of said heat exchanger tube.
According to the present invention, said twisted baffles are integrated with the inner surface of said heat exchanger tube, so that said twisted baffles are not easy to be damaged. Therefore, said heat exchanger tube can always work well and be reliability during heat exchanging, and be able to work for a longer time.
According to the method for making the heat exchanger tube according to the present invention, the heat exchanger tube is made by so called precision casting, therefore, it can make sure that the surface of the twisted baffle of the heat exchanger tube is smooth enough, the inner wall of the heat exchanger tube is finished enough, and the errors in dimension and in geometrical form are small enough.
As the surface of the twisted baffle of the heat exchanger tube is smooth enough, the resistance to the flow of the in-process materials can be minimized, therefore, no eddy can be formed in any point of the in-process materials passage. With this reasons, it is more possible to prevent coking.
According to the method for making the heat exchanger tube according to the present invention, the heat exchanger tube is made by so called casting, therefore, it can make sure that the heat exchanger tube can be made in a simple, easy and low cost way. Moreover, as the heat exchanger tube is made by casting, the heat exchanger tube is weldable so as to be able to be welded into the furnace tube, in this way, the heat exchanger tube can be connected to the furnace tubes in a simple, easy and low-cost way.
According to the present invention, in the cracking furnace tube, any heat exchanger tube with the twisted baffle(s) is axially provided between two furnace tubes located outside the two ends thereof respectively, and any two of said heat exchanger tubes are separated from each other in the furnace tube, that is to say, said heat exchanger tube with the twisted baffle(s) is only provided in one or more portions of the furnace tubes, so that the total length of all said heat exchanger tubes with the twisted baffle(s) is only a small part of the entire length of the furnace tubes. Therefore, the resistance to the flowing in-process materials can not be increased greatly, so that the in-process materials can not only go forward while being in a helical motion itself so as to increase the efficiency of the heat transfer, but also not notably make the flowing speed of the in-processing flow slower.
With the help of said twisted baffle inside said heat exchanger tube, said twisted baffle directs the in-process materials laterally away from the center of said heat exchanger tube while flowing forward, so that the in-process materials strongly spray onto the inner surface of said heat exchanger tube, making the thickness of the boundary statically flowing layer on the inner surface of said heat exchanger tube much thinner, so that the heat resistance of the boundary layer on the inner surface of said heat exchanger tube is much smaller, therefore, the flowing speed of the in-process materials can suitably be faster.
In the present invention, as said heat exchanger tube has said twisted baffle therein, resulting in a tendency to increase the resistance to the flowing in-process materials, however, the negative influence resulted from this tendency is much smaller than the positive influence come from the efficiency of the heat transfer being increased. On the other hand, the temperature of the flowing in-process materials flowing near the inner surface of said heat exchanger tube is lowered. This in turn make it more possible to prevent coking on the inner surface of said heat exchanger tube, therefore, with the help of said heat exchanger tube according to the present invention, not only the output of the desired chemical product can be increased, but also the period for clearing the coking can be lengthen.
By means of adding said heat exchanger tube with the twisted baffle(s) into the furnace tubes, the temperature on the inner surface of said the radiation heating furnace tube of the cracking furnace tube is lowered, said the radiation heating furnace tube of the cracking furnace tube can be used for a longer time.
With the same reasons, by means of adding said heat exchanger tube with the twisted baffle(s) into the tubular heat furnace, the efficiency of the heat transfer of the tubular heat furnace can be increased, and more flowing in-process materials can be gone through.